Heretofore, there has been known a metal gasket which is interposed between joint surfaces of a cylinder block and a cylinder head to fasten the cylinder block and the cylinder head by a fastening bolt so as to seal the joint surfaces. It is known that when this kind of metal gasket is interposed between the cylinder head and the cylinder block to fasten them by the fastening bolt, a fastening load of the position adjacent to the bolt hole into which the fastening bolt is inserted enlarges, and the fastening load of the position away from the bolt hole lessens, that is, the fastening load around the cylinder hole becomes nonuniform.
To solve such a problem, heretofore, a metal gasket has been proposed, in which a pair of bead plates having a cylinder hole corresponding to a cylinder bore and a full bead surrounding the cylinder hole, an intermediate plate interposed between this pair of bead plates, and a shim plate interposed between one bead plate and the intermediate plate inside the full bead are provided, and the thickness of a contact portion to contact the shim plate in the intermediate plate is set thin at the position adjacent to the bolt hole into which the fastening bolt is inserted, and is set thick at the position away from the bolt hole (Patent Document 1).
According to the above described metal gasket, since the thickness of the contact portion at the position adjacent to the bolt hole whose fastening load increases is made thin, the fastening load of that portion can be made small, whereas the thickness of the contact portion at the position away from the bolt hole whose fastening load decreases is made thick, and therefore, the fastening load of that portion can be made large. Consequently, if the thickness of the contact portion is changed so as to be most appropriate, the fastening load around the cylinder hole can be made uniform.
Patent Document 1: Japanese Patent Application Laid-Open No. H11-201285
However, in the conventional metal gasket disclosed in the Patent Document 1, since a shim plate is provided inside a full bead, effective utilization of the elasticity of the full bead has been interfered by a thickness portion of the shim plate as a result. In other words, if the shim plate is not provided, the full bead can utilize the whole area of the height of the full bead as an elastic area, but if the shim plate is provided, the full bead has the elastic area reduced by the thickness portion of the shim plate, and consequently, effective utilization of the elasticity of the full bead is interfered by a thickness portion of the shim plate. As a result, a follow-up performance to head amplitude at the time of combustion is lowered, thereby bringing about a lowering of sealability.
The present invention has been made in view of the above described problem, and an object of the invention is to provide a metal gasket capable of enhancing the sealability by effectively utilizing the elasticity of the full bead, and eliminating the fluctuation of fastening load distribution around the cylinder hole to suppress the local concentration of the fastening load, thereby to enhance durability by suppressing damages of the full bead and the shim plate in the metal gasket applied by interposing into joint surfaces with a cylinder block and a cylinder head.